once and for all

1. 永远地；一次了结地
   John's back home once and for all.

   约翰回到家，再也不走了。

once in a while

1. 有时
   He went to see them once in a while.

   他有时去探望他们。

once more/again

1. 再一次
   John's back home once more.

   约翰又回家了。

once or twice

1. 几次
   I've been there once or twice.

   我到过那里几次。

once too often

1. 又一次
   He exceeded the speed limit once too often.

   他又超速行驶了。

all at once

1. 突然
   He left all at once.

   他突然离开了。

at once

1. 马上；立刻
   Do it at once!

   马上就做！

for once

1. 仅此一次
   For once he was telling the truth.

   他就这一次说了真话。

• all at once
  
  1. suddenly.
  2. all at the same time.
• at once
  
  1. immediately.
  2. simultaneously.
• for once (or this once)
  on this occasion only.
• once again (or more)
  one more time.
• once and for all (or once for all)
  now and for the last time; finally.
• once (or every once) in a while
  occasionally.
• once or twice
  a few times.
• once upon a time
  at some time in the past.